Of particular interest to the invention disclosed herein is the transport stream as defined in Annex D of the “ATSC Digital Television Standard” published by the Advanced Television Systems Committee in 1995 as its document A/53. This standard defines the broadcasting of digital television (DTV) signals within the United States of America and is referred to in this specification simply as “A/53”. Section 4 of Annex D titled “Transmission Characteristics for Terrestrial Broadcast” is incorporated herein by reference. The Transport Stream (TS) used in DTV broadcasting utilizes (207, 187) Reed-Solomon forward-error-correction (R-S FEC) coding of segments of randomized data, each consisting of one-hundred-eighty-seven bytes mapping the final one-hundred-eighty-seven bytes of an MPEG-2-compliant data packet that was randomized. The twenty bytes of R-S FEC coding for each data segment is appended to its end to stretch its length to two-hundred-and-seven bytes. The R-S FEC coding is followed by convolutional interleaving and then by 2/3 code rate trellis coding. The trellis-coded symbols are then mapped to an eight-level modulation signal used in the generation of a vestigial-sideband amplitude-modulated radio-frequency carrier signal referred to as an 8VSB signal.
After the “ATSC Digital Television Standard” was established in 1995 reception of terrestrial broadcast DTV signals proved to be problematic, particularly if indoor antennas were used. A variety of schemes have been proposed to time-division multiplex segments of more robust data with segments of ordinary 8VSB data. These schemes involve reduction of the 8VSB symbol alphabet, or further coding of the 8VSB signal, or a combination of both those approaches. These schemes seek to preserve the trellis coding scheme specified by A/53 throughout each data field, so as to avoid disrupting the reception of segments of ordinary 8VSB data by legacy DTV receivers. At the same time, these schemes have sought to cause those legacy receivers to disregard the segments of robust data. The approach taken in the prior art has been to confine the robust data to the payload portions of MPEG-2-compliant data packets, with the 13-bit packet-identifier (PID) portions of the packet headers being used to identify the payload portions as containing robust data. This approach is taken in and in the E-8VSB standard for robust DTV broadcast transmission recently adopted by the ATSC. The problem with this approach is that MPEG-2-compliant data packets recoded to halved code rate or to quartered code rate cannot be fitted into an integral number of data segments. Accordingly, undesirably elaborate measures must be taken for time-division multiplexing robust data transmissions with segments of ordinary 8VSB data.
U.S. patent application Ser. No. 10/885,460 titled “REED-SOLOMON CODING MODIFICATIONS FOR SIGNALING TRANSMISSION OF DIFFERENT TYPES OF DATA PACKETS” was filed 6 Jul. 2004 by A. L. R. Limberg and subsequently abandoned. That application teaches that different types of data transmitted via a DTV broadcast signal can be distinguished from each other by the types of R-S FEC coding they respectively use. That application describes 187-byte chunks of data associated with robust transmission being subjected to modified (207, 187) R-S FEC coding. These 187-byte chunks can be halves of 187-byte MPEG-2-compliant data packets recoded to halved code rate or can be quarters of 187-byte MPEG-2-compliant data packets recoded to quartered code rate. In either case the recoded MPEG-2-compliant data packets are easily fitted into an integral number of data segments.
U.S. patent application Ser. No. 10/885,460 further points out that a (207, 187) R-S FEC codeword as prescribed by the “ATSC Digital Television Standard” can be modified by complementing prescribed ones of its bits in eleven or more of its bytes, so as to introduce byte errors that exceed the byte-error-correction capabilities of the (207, 187) R-S FEC decoders in legacy DTV receivers. The modified (207, 187) R-S FEC codewords are decoded in new receivers by complementing the prescribed bits of each modified (207, 187) R-S FEC codeword to restore the original (207, 187) R-S FEC codeword, which is then decoded conventionally. By prescribing different bits to be complemented in each (207, 187) R-S FEC codeword, many different modified (207, 187) R-S FEC codewords can be generated, each distinguishable from all the others.
U.S. patent application Ser. No. 10/885,460 points out that the (207, 187) R-S FEC code prescribed by the “ATSC Digital Television Standard” is a shortened 255-byte R-S FEC code, the 48 extra “virtual” bytes of which are null bytes in the check equations. The parity bytes in the (207, 187) R-S FEC code used for the data field segments presume certain values for the null bytes, the presumption being that all bits in each null byte are ZEROes. If at the transmitter the R-S FEC coding of selected segments were done with different “virtual” bytes, the parity bytes would all be changed. Legacy DTV receivers presuming the 48 “virtual” bytes were null bytes when decoding the modified shortened R-S FEC code would find the segment to contain uncorrectable byte errors and would discard the MPEG-2-format packet recovered from that segment. In new receivers decoding of the (207, 187) shortened R-S FEC code can initially be done presuming the 48 “virtual” bytes are null bytes, to determine whether or not a data field segment is a correctable data segment. If it is not, the check equations can be extended to include different “virtual” bytes associated with a particular form of robust transmission, to determine whether or not a data field segment is a correctable segment associated with that particular form of robust transmission.
U.S. patent application Ser. No. 09/840,481 filed 23 Apr. 2001 by A. J. Vigil, Jr. and M. A. Belkerdid and now abandoned was published 30 Jan. 2003 with publication No. 2003-0021341. Their patent application described the introduction of data segments with prescribed training symbols into the data fields supplied for convolutional interleaving and subsequent 2/3 code rate trellis coding. These prescribed training symbols were introduced to aid the adaptation of the weighting coefficients in the kernels of digital filters used for channel equalization, presuming that autoregression methods would be used for adapting those weighting coefficients. Paragraph 0068 of publication No. 20030021341 points out that the segments composed of prescribed training symbols were not valid (207, 187) R-S FEC codewords nor were they correctable to be so by the R-S FEC codeword decoders in legacy DTV receivers. Consequently, legacy DTV receivers would in effect discard these segments composed of prescribed training symbols by not time-division multiplexing the initial 187 bytes of each of them into the respective transport streams reproduced by those receivers.
U.S. patent application Ser. No. 10/955,212 filed 30 Sep. 2004 by A. L. R. Limberg and titled “TIME-DEPENDENT TRELLIS CODING FOR MORE ROBUST DIGITAL TELEVISION SIGNALS” was published 7 Apr. 2005 with publication No. 2005-0074074. That application observes that many data segments containing robust data will not be mistaken by legacy receivers as being (207, 187) R-S FEC codes that appear to free of byte error or can be corrected to appear so. Accordingly, legacy DTV receivers should not forward those data segments to packet decoders. The problem remaining is that there is the possibility that some data segments containing robust data can be mistaken by legacy receivers as being (207, 187) R-S FEC codes that appear to free of byte error or can be corrected to appear so. Patent application Ser. No. 10/955,212 suggests that data segments containing robust data be evaluated to determine whether such mistake is likely and, if so, that their projected position in the data field be altered to make such mistake unlikely. Changing the position of the data segments containing robust data will change the randomization of the data. FIG. 10 of application Ser. No. 10/955,212 shows a routine to validate the insertion of data segments using a restricted symbol alphabet into the time-division-multiplex signal supplied for convolutional interleaving and trellis coding. The inventor sought an alternative procedure to avoid legacy DTV receivers mistaking data segments containing robust data for (207, 187) R-S FEC codewords that are correct or capable of correction, which alternative procedure would substantially reduce or completely eliminate the need to reposition those data segments.
U.S. patent application Ser. No. 10/955,212 discloses that the various different methods of R-S FEC coding described by U.S. patent application Ser. No. 10/885,460 for distinguishing between different types of signal can be adapted to provide such alternative procedures. U.S. patent application Ser. No. 10/955,212 extended the concept of causing legacy DTV receivers to discard certain 207-byte segments in effect. The 207-byte segments contained random data generated from the information content to be transmitted, rather than just prescribed symbols of which a DTV receiver could have a priori knowledge to improve adaptation of the weighting coefficients of digital filtering the receiver used for channel equalization.
U.S. Pat. No. 5,377,207 granted 27 Dec. 1994 to M. Perlmanand is titled “Mappings between codewords of two distinct (N, K) Reed-Solomon codes over GF (2.super.J)”. Perlman points out that a Berlekamp alternative (N,K) R-S code is orthogonal to conventional-architecture (N, K) R-S code and that substantially the same hardware can be used for decoding either type of (N, K) R-S code. It is known theoretically that other “orthogonal” R-S codes that sustain error correction besides the type described by Berlekamp exist. They differ from conventional-architecture R-S code in that their Galois fields are generated by primitive field generator polynomials with zero coefficient placements different from those in the primitive field generator polynomial of the conventional-architecture R-S code. U.S. Pat. No. 5,490,154 granted to R. Mester on 6 Feb. 1996 and titled “Method of and circuit arrangement for decoding RS-coded data signals” indicates that Philips used substantially the same hardware for decoding either of the EBU D1 and ISO R-S codes employed in magnetic tape recording. With hindsight gained from U.S. patent application Ser. No. 10/885,460, it appears that previously known “orthogonal” R-S codes that sustain error correction can be used to distinguish different types of data transmitted via a DTV broadcast signal. With hindsight gained from U.S. patent application Ser. No. 10/955,212, it appears that previously known “orthogonal” R-S codes that sustain error correction can be used to encode data segments containing robust data to avoid legacy DTV receivers mistaking such segments for (207, 187) R-S FEC codewords that are correct or are capable of correction.
U.S. patent application Ser. No. 10/733,645 filed 12 Dec. 2003 for A. L. R. Limberg and titled “ROBUST SIGNAL TRANSMISSIONS IN DIGITAL TELEVISION BROADCASTING” was published 25 Nov. 2004 with publication No. 2004-0237024. U.S. patent application Ser. Nos. 10/733,645, 10/885,460 and 10/955,212 describe “transverse” Reed-Solomon forward-error-correction (or TRS FEC) coding being done on symbols within paths that cross the data segments in a data field yet before the bytes in those segments are subjected to convolutional interleaving. Such TRS FEC coding provides additional capability for correcting byte errors and can be used together with the inventions disclosed in this specification. In this specification R-S FEC coding that is done on the symbols within a data segment before its bytes are subjected to convolutional interleaving is referred to as “lateral” Reed-Solomon forward-error-correction (or LRS FEC) coding. The reader is advised that the numbering of the various types of LRS FEC coding described in this specification is independent of the numbering of types of LRS FEC coding in other documents. The parity bytes of the TRS FEC coding can be provided LRS FEC coding of a type distinguishable from the LRS FEC coding used for other information.
U.S. patent application Ser. No. 11/119,662 filed 2 May 2005 by A. L. R. Limberg and titled “DIGITAL TELEVISION SIGNALS USING LINEAR BLOCK CODING” describes the code rate of DTV data being halved using linear block codes. E.g., the linear block codes are used with restricted symbol alphabets to quarter the 2/3 code rate of ordinary trellis coded 8VSB DTV signal. U.S. patent application Ser. No. 11/119,662 prescribes modifying data segments containing robust data that could otherwise be mistaken for (207, 187) R-S FEC codewords by legacy DTV receivers. This is done by complementing all the bits in the final twenty bytes of the data segment if the robust data are transmitted using pseudo-2VSB modulation. Or, this is done by complementing the Z2 or X2 bits in the final twenty bytes of the data segment if the robust data are transmitted using prescribed-coset-pattern modulation (PCPM) or another coding scheme in which modification of Z1 bits is avoided.